Yeah, that information is not provided, just the electrical power, a lot of which I'm sure is heat.

This is the problem. Without knowing the efficiencies of the different types of lights it would be difficult to compare them since the light output of each isn't readily available. I assume the LEDs are more efficient in converting electrical power to light power, so my guess is that an 11.8 watt LED would put out a lot more light than a 14 watt fluorescent – much more than just the 2.2 watt difference.

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if you can, test different depths with a PAR meter

Yes, directly measuring the amount of light energy would be the way to go and a PAR meter would give the amount of light energy, at least the amount that reaches a surface. But you would first need to measure the output of a fluorescent fixture that is known to produce good results in order to match that with the LEDs.

Yes, directly measuring the amount of light energy would be the way to go and a PAR meter would give the amount of light energy, at least the amount that reaches a surface. But you would first need to measure the output of a fluorescent fixture that is known to produce good results in order to match that with the LEDs.

to compare, you can find par readings online, esp on lighting for saltwater tanks. it prob. wouldnt be practical to build lights then pay to buy a PAR meter but if you could borrow someones that would be the best way to test what youve built.

If I do it I will experiment on my 20 gallon, probably get three of the LEDs. If I can do it for $50 that isn't bad compared to buying a different fixture and tubes. I already have a single 14W T8 8000K fixture, but that's certainly 'low light' and I'd rather have 'moderate'. And if the plants grow well under it... so much the better =)

If not ... well, I'll have a really bright light strip I can use to light a room hehe.

yep a DIY LED rig would certainly work. At least you are down the right path using high intensity LEDs. They are still too rich for my blood lol, even DIY. Prefer to stick with cheaper DIY fluorescent fixtures. Those LEDs put out some serious heat though, they NEED to be heat sinked or they will burn themselves out.

I've used my camera in manual mode as a cheap way to gauge the intensity of light, but you can really only compare two fixtures against each other.

yep a DIY LED rig would certainly work. At least you are down the right path using high intensity LEDs. They are still too rich for my blood lol, even DIY. Prefer to stick with cheaper DIY fluorescent fixtures. Those LEDs put out some serious heat though, they NEED to be heat sinked or they will burn themselves out.

I've used my camera in manual mode as a cheap way to gauge the intensity of light, but you can really only compare two fixtures against each other.

Yep, I've already run through all the calculations for a heat sink and have parts selected

Trying to decide how I want to handle the large tolerances in the forward voltage, power supply, and current limiting resistor. I'd like to order everything at once to save shipping costs, but may be easier to buy the resistors after the others are known.

I'd kill for a local electronics store, or even a radio shack of a couple decades ago.

The more I think about this, the more it turns into a thermal nightmare :X The LED was simple, I knew a heatsink was needed for it and found a simple solution.

The rest of it though...ugh. Sometimes writing it out helps the brainstorming process.

There are pretty much two ways to drive an LED. Either you put a resistor in series to limit the current, or you use a constant current source.

A resistor is the time tested ultra simplistic and cheap method, after all the only part is a single resistor which are really easy to find and cost pennies. The issue with them is the current will vary widely depending on a number of variables. Current is what determines how 'bright' the LED is. Too small and it will be dim, too high and it will burn the device out.

A constant current source, as it implies, ensures that a constant current goes through the LED. This means its 'brightness' is a constant, and known. This requires a few more parts and there are a couple methods of achieving this cheaply.

Both methods have a power dissipation problem though. In both cases you have a resistor that must take the full current load of the LED. For the particular one I'm looking at it is 350mA which while sounds small is actually quite high. In the resistor only option it must dissipate ~2W of power (your typical surface mount resistors that almost everything uses this day can only do 1/8 W, the older through hole ones are 1/4 W). 4W+ resistors do exist (you want to be about 2 times higher than your power for reliability reasons), but they're large-ish and get very hot and of course cost more (but still not unreasonable).

For the constant current option, it is a transistor that takes the brunt of that power instead of the resistor. Heatsinks for transistors exist, but still you're looking at the thing getting ~50 deg C (122 F) higher than ambient, so you're looking at around 200 deg F. Ouch!

I'm not concerned about the parts themselves, they can take the heat, more having things that hot exposed where fingers/arms may brush against. Building an enclosure, and while your at it putting a small fan in, would make everything better but that adds yet more cost.

Debating if it is worth just buying a pre-made constant current LED driver. They are more sophisticated in how they work to get higher efficiency (thus lower power dissipation), but cost about $10 each (would need three for my purpose). They too could be custom built, but would not see any cost savings. They would make the whole project cost upwards of $100 at which point they are not much savings over commercial products.

I think you would be better off with drivers since thats what I normally see used. You have kinda covered all the reasons why I haven't messed with high powered LEDs. They are cool, but even the cost of DIY I don't find competitive to fluorescents. Then LEDs have their own draw backs as well.

Well, I decided to take the plunge and go for it. Why not? In any case, it should be a good learning experience.

Parts were a little more than expected, but I ordered some extras on a lot of it 'just in case'.

I went with an LED that puts out 685 lumens, which is just shy of what a GE 18" T8 Daylight flourescent bulb. The color spectrum will be a little different (the GE is 6500K or 6700K I forget) while the LED is a little warmer at 5665K.

I plan on using three of them, but under driving them. My goal is to be roughly equivalent to a dual T8 fixture. They have a viewing angle of 120 degrees, so if my trigonometry is still good (questionable...) a single one of these should light the entire bottom surface of the tank. But, of course, that would be like a 'spotlight' which can be a cool effect on its own, but I want it to be a little more evenly spread.

So, what is the cost? $70.26, including tax and shipping. $20 more than I wanted, but I will have enough left over to almost have enough to make 5 LEDs. The most expensive part was actually the sheet of plastic I'm going to use to assemble it to. If you already have a wood canopy (or old fixture to cannibalize) that's $12.50 saved right there. Without that, and the spare parts, the cost would have been around the estimated $50.

And if it dosen't work well for growing plants ... Well, I can have a very bright light strip installed in the basement laundry room ;)